1. Field of Invention
The present invention relates to power amplifiers with output voltage compensation, and more particularly to a power amplifier utilizing current feedback to provide output voltage compensation.
2. Discussion of Related Art
FIG. 4 is a functional block diagram of a conventional power amplifier 400 that receives an input voltage Vin and provides a voltage compensated output voltage Vout to a load 490. As depicted in FIG. 4, the power amplifier 400 includes an input (summing) stage 410, a voltage gain stage 420, a power gain stage 440, and a voltage feedback compensation stage 460 connected in the manner shown. The output voltage Vamp of the power amplifier 400 at output node 475 is sensed by the voltage feedback compensation stage 460, and a voltage feedback compensation signal that is based upon that sensed output voltage is provided to the input (summing) stage 410. Negative feedback is used to control the amount of amplification provided by the voltage gain stage 420 (and power gain stage 440) based upon a difference between the input voltage Vin and the voltage feedback compensation signal (which is itself based upon the output voltage Vamp of the power amplifier 400). Typically, there is some series output impedance 480 that is associated with circuitry to which the power amplifier 400 is connected, the load, or both.
Although the power amplifier 400 is capable of providing a compensated output voltage Vout, it may be relatively slow in responding to changes in the input resistance of the load 490. For example, it is known that the input resistance of certain loads, for example, integrated circuits, may change very rapidly. In response to such rapid changes in the input resistance of a load, the output voltage Vamp provided by the power amplifier 400 and the output voltage Vout provided to the load may fail to be adequately compensated.
FIG. 5 illustrates the voltage and current characteristics of a conventional power amplifier, such as that depicted in FIG. 4, in response to a change (specifically, an increase) in the input resistance of a load. Due to the sudden increase in resistance of the load (which may be due, for example, to the load entering a sleep mode, or other effective high impedance, low current state), the output current, lout, provided to the load may drop to a negligible value in a very short period of time (e.g., as little as 5 ns). Because the output voltage Vamp is used for voltage feedback compensation, and the output voltage Vout=Vamp−Iout*(Zo(s)), the output voltage Vout provided to the load will see a rapid increase in value. Over time, the negative feedback provided by the voltage feedback compensation will eventually adjust the voltage output of the amplifier to the change in resistance of the load, but such adjustment may not be adequate to prevent damage to the load, etc.